I have a 2001 s4... its apr tuned stage 2. Its been stage two for about 6 months now and my check engine light has been on ever since i got rid of the cats with the apr down pipes... but all of a sudden my car has been acting weird around 4000 rpm it acts like the car stalls out then it kicks in and goes... so thinking it was my downstream oxygen sensors I bought the vast o2 spacers but the car still acted up after installing them... so I got a friend of mine to check it out with his code reader and it told me that it wasn't my downstream o2s at all its sayin my upstream sensors are bad... which I found out one was because of some heat damage to the connector... so what I need is two new upstream ( bank 1 sensor 1 and bank 2 sensor 1) o2 sensors... can anyone help me out? Has anyone else had this same problem??? Thnx